This invention relates to a means of labeling nucleic acids, preferably DNA. More particularly, this invention is directed to a process for preparing labeled nucleic acids by use of alkylating intercalators containing a label moiety, where the word label is intended to include moieties which may be detected both directly and indirectly. In addition, this invention relates to a means for detecting the presence of a nucleic acid sequence such as a gene using a hybridization probe containing a complementary nucleic acid sequence.
In biomedical research and recombinant DNA technology it is often useful to have indicator probes which allow the user to detect, monitor, localize or isolate nucleic acids when present in any amount. DNA hybridization probes, for example, contain a nucleic acid sequence complementary to the nucleic acid sequence or to the gene to be detected. Such probes have been used to detect the presence of genes coding for antigens responsible for graft rejection, such as human leukocyte antigen (HLA), or genetic disease, such as sickle cell anemia. For example, Sood et al., PNAS, 78, 616-620 (1981) describe the isolation of cDNA clones for HLA-B antigens. These clones were prepared by synthesizing cDNA from an mRNA mix containing mRNA coding for the desired HLA antigen, inserting the cDNA into a vector, transforming a bacterial host and isolating transformant clones that contain the desired DNA segment by probing with an oligonucleotide probe that is specific for the desired DNA sequence. Ploegh et al., PNAS, 77, 6081-6085 (1980) have also reported cloning a cDNA probe for an HLA gene sequence. In addition, U.S. Pat. No. 4,358,535 to Falkow et al. describe a method for detecting infectious disease-causing microbes using labeled nucleotide probes complementary to nucleic acid contained by the pathogenic microbe. Until recently, the materials most sensitive and therefore useful for this purpose were radioactively labeled nucleic acids such as those labeled with isotopes of, e.g., hydrogen (.sup.3 H), phosphorus (.sup.32 p) or iodine (.sup.125 I).
Such radioactive compounds, however, suffer from various drawbacks, including extensive safety precautions, expensive equipment, health-monitoring services and waste treatment, and high usage costs due to the instability of the materials. Therefore, there is an increasing incentive to search for suitable nonradioactive labels for nucleic acids which would provide sensitive probes.
Already known is that haptens can initiate an immune response if bound to a carrier, so as to be useful for labeling and identification. Thus, for example, hapten-labeled DNA can be detected with antibodies.
It is also known that biotin interacts with streptavidin or avidin, a 68,000 dalton glycoprotein from egg white, to form a tightly held non-covalent complex which has been recently used to develop methods for visually localizing specific proteins, lipids or carbohydrates on or within cells. For example, Manning et al., Chromosoma, 53, 107 (1975) have determined the chromosomal location of ribosomal genes by election microscopy using a biotinized protein, cytochrome C, chemically crosslinked to RNA as a hybridization probe. Langer et al., Proc. Natl. Acad. Sci. USA, 78, 6633-6637 (1981) describe a method for labeling DNA by enzymatic incorporation of nucleotide analogs containing functional groups such as biotin via DNA polymerase I, and Leary et al., Proc. Natl. Acad. Sci. USA, 80, 4045-4049 (1982) have used this method to label DNA probes with biotinylated nucleotides. European Patent Publication No. 0,063,879 published Nov. 3, 1982 to D. Ward et al. describes nucleotide derivatives which contain biotin, iminobiotin, lipoic acid and other labels attached covalently to the pyrimidine or purine ring which will interact with proteins such as avidin or antibodies. When biotin is bound specifically by an avidin-linked enzyme complex, detection is seen as a color change in a chromogenic substrate. When an avidin-alkaline phosphatase complex is used to detect biotinylated DNA probes after hybridization, sensitivity has been shown to approach that of autoradiography used to detect .sup.32 p labeled probes.
The method of Ward et al. for nonradioactive labeling results in labeling of the hybridizing region of the probe, thus causing significant interference with hybridization.
U.S. patent application Ser. No. 444,438 filed Nov. 24, 1982 to Letsinger et al. describes bifunctional intercalaters containing a phenanthridium moiety as an agent for introducing markers (e.g., fluorescent probes) at specified regions in polynucleotides.
Various methods exist for attaching chemical labels to DNA. For example, it is well known how to attach chemical moieties to pyrimidine and purine rings using an acetoxymercuration reaction whereby covalently bound mercury atoms are introduced into the 5-position of the pyrimidine ring, the C-8 position of the purine ring, or the C-7 position of a 7-diazapurine ring. European Patent Publication 0,063,879 supra describes the preparation of a nucleotide derivative by a process where a mercurated intermediate is formed which reacts with a reactive chemical moiety which may be the label or which then reacts with the label compound. In these methods, the labeled nucleotide is then incorporated into DNA so that the DNA is labeled.
Methods also exist for studying the structure of DNA. For example, psoralens, which are a class of planar furocoumarin molecules capable of intercalating into double-stranded DNA in the presence of single-stranded DNA, will covalently bond to and crosslink DNA when activated by long-wave (&gt;350 nm) UV light. Covalent bonding involves a two-step process: (1) intercalating the planar-structured psoralen between the base pairs in the double helix structure of the nucleic acid to produce a psoralen-nucleic acid complex and (2) irradiating the complex with light of the proper wavelength to form covalent bonds between the psoralen molecules and pyrimidine nucleotides which occur as integral entities of nucleic acid strands.
This covalent bonding enables the study in vivo of secondary structures of DNA such as packaging of nucleic acid within viruses. Use of 4'-adducts of 4,5',8-trimethylpsoralen to bond DNA covalently is described in U.S. Pat. No. 4,124,598 to Hearst et al. Hearst, Rapoport and others have extensively studied the incorporation of psoralens into DNA and RNA. Brown et al., Gene, 20, 139-144 (1982) teaches stabilizing radioactive RNA-DNA hybridization probes using trimethylpsoralen.
Saffran et al., Proc. Natl. Acad. Sci. U.S.A., 79, 4594-4598 (1982) describes site-directed psoralen crosslinking of DNA to enable structural analysis using a psoralen derivative containing a thiol group. In this process the plasmid DNA molecule has mercurated nucleotides incorporated near a restriction site so that the psoralen is directed to the bases through a Hg-S linkage.
In addition to their use in studying nucleic acid secondary structure, commercial applications of the psoralen derivatives include their use in treating certain dermatological disorders and for viral inactivation to produce vaccine.
Use of mercurated compounds in reaction sytheses involves extra expense and necessitates safety precautions in view of the toxicity of mercury.
Another use for compounds which label DNA is in chromosome banding or staining. An example described in the literature is the use of the Giemsa reagent to stain regions or bands of chromosomes differentially, as described in the article by V. G. Dev et al., Lancet (England) 1, 1285 (June 10, 1972). Because chromosomes have characteristic banding patterns, this procedure can be used to distinguish chromosomes. This ability to distinguish chromosomes has been very useful in the study of chromosome anomalies. For example, Down's syndrome can be diagnosed by determining that the individual is trisomic for chromosome 21.